Worryingly, users' passwords were not encrypted in any way — leaving users extremely vulnerable to hackers.

Data breaches like these are valuable to hackers and identity thieves because they can use the login details, if not properly secured, to try and log into other services that users have an account with. If you re-use a password, every one of your accounts, from banking to social networks, can end up compromised as a result of a successful hack on just one.

As such, this breach will remain useful to criminals even though it's more than four years old — because many people re-use the same password over and over, never changing it. It's for this reason that security experts recommend you use a strong, unique password for every site or service you're registered with, using a password management app to store them if necessary. If you don't, it could come back to bite you — years later.

All in all, LeakedSource says, 98,167,935 user accounts have been leaked. Rambler.ru did not immediately respond to a request for comment.